The present invention relates to a MOS type non-volatile semiconductor memory with a floating gate electrode.
An electric charge injection method from a semiconductor substrate to a floating gate electrode in relation to a conventional channel injection-type non-volatile semiconductor memory is explaned with FIG. 1a, FIG. 1b, FIG. 1c and FIG. 2. FIG. 1a is a plan view of a conventional lucky electron injection type non-volatile semiconductor memory, FIG. 1b is a sectional view taken on line A--A' of FIG. 1a, and FIG. 1c is an equivalent circuit of this memory.
The following is an electric carrier injection method of a N-type memory transistor. In FIGS. 1a, b, a N.sup.+ type source region 2 and drain region 3 are formed at some space adjacent to a surface of a P-type semiconductor substrate 1, and a second gate insulator 4 and a third gate insulator 5 are each formed on two channel regions l.sub.1 and l.sub.2 between the source region 2 and the drain region 3, and a floating gate electrode 7 is formed on the third gate insulator 5 and a first gate insulator 6 provided on the drain region 3, and then a select gate electrode 8 is formed on the second gate insulator 4.
A write operation (electric carrier injection from the semiconductor substrate I to the floating gate electrode 7) is done in the following manner.
The floating gate electrode 7 is capacitance coupled with the circumferential electrode and regions as shown FIG. 1c. In FIG. 1c, V.sub.SG is the voltage of the select gate electrode 8, V.sub.D is the voltage of the drain electrode 3a, V.sub.F is the voltage of the floating gate electrode 7 and, Vsub is the voltage of the semiconductor substrate 1. Usually, Vsub=0 because the voltage of the semiconductor substrate 1 becomes the ground potential for all electrodes. C.sub.SG is the capacitance between the floating gate electrode 7 and the select gate electrode 8, and C.sub.D is the capacitance between the floating gate electrode 7 and the drain electrode 3a, and Csub is the capacitance between the floating gate electrode 7 and the semiconductor substrate 1.
C.sub.D C.sub.SG, Csub are realized clearly due to the structure as shown in FIGS. 1a, b.
Therefore, the following equation is given always. EQU V.sub.F .perspectiveto.V.sub.D ( 1)
By applying large drain voltage, the channel region l.sub.2 below the floating gate electrode 7 can be inverted strongly and the surface potential of the channel region l.sub.2 is almost the same as the drain voltage V.sub.D. Namely, the band structure of the sectional view on line c--c' of FIG. 1b can be drawn as shown FIG. 2. The potential distribution is bent at the surface of channel region l.sub.2.
According to the equation (1), the surface potential .phi..sub.S is given approximately by the equation (2). EQU .phi..sub.S .perspectiveto.V.sub.D ( 2)
Namely, flowing the forward current from the source region 2 to the semiconductor substrate 1 when the channel region l.sub.1 is not inverted, a part of the forward current can be injected from the semiconductor substrate 1 to the floating gate electrode 7 along an arrow D of FIG. 2. Such electric carriers injection method is called lucky-electron injection.
To inject the carriers to the floating gate electrode 7, the following equation must be realized clearly due to FIG. 2. EQU .phi..sub.S &gt;.phi..sub.C ( 3)
where .phi..sub.C is the barrier energy of the semiconductor substrate 1--the third gate insulator 5. When the semiconductor substrate 1 is made of Si and the third gate insulator 5 is made of SiO.sub.2, .phi..sub.C is about 3.2 V.
Normally, the electrons in semiconductor substrate 1 decrease in energy due to collisions during the bulk electron flow to the floating gate electrode 7. The electrons with an energy which satisfies eq.(3) at the surface of the channel region l.sub.2 can be injected to the floating gate electrode 7. In the case of lucky-electron injection type memory because the doping density of the semiconductor substrate 1 is high and the width of the depletion layer at the surface is short, the energy loss due to collision is reduced.
FIG. 3 shows the minimum drain voltage V.sub.WO to inject electrons from the semiconductor substrate 1 to the floating gate electrode 7 (minimum write voltage) as a function of the doping density of the semiconductor substrate 1 in the case of that the gate insulator is 200 .ANG. SiO.sub.2.
The minimum write voltage V.sub.WO can be decreased by increasing the doping density N.sub.A. FIG. 4 shows the write characteristics of a memory as functions of the length of channel region l.sub.1. The threshold voltage shift of the channel region l.sub.2 is the difference between the initial threshold voltage and the threshold voltage after write. And I.sub.D /Iinj is the ratio of drain current I.sub.D to injection current Iin; flowed from the source region 2 to the semiconductor substrate 1. The electrons injected to the floating gate electrode 7 increase with drain current. .DELTA.V.sub.T increases with the injected electrons and I.sub.D /Iinj decreases with useless injection current.
The injection efficiency of electrons depends strongly on the length of channel region l.sub.1 as shown in FIG. 4. I.sub.D /Iinj decreases with the length of channel region l.sub.1. Therefore the electrons which flow from the source region 2 to the semiconductor substrate 1 can be injected efficiently to the floating gate electrode 7 in the case of a memory with a short channel region l.sub.1. But few electrons can be injected to the floating gate electrode in the case of a memory with a long channel region l.sub.1.
Therefore, reciprocal action between multiple memories is strong when the distance between the memory and a next memory is short because the inject current can flow to the nonselected memory.
As the above explanation, a bipolar-lucky-electron injection type memory with forward current has the following weak points.
(1) Two voltage sources with different polarity are needed to write information in each memory because the write operation needs the forward current. PA0 (2) A large current is needed to write information in each memory because the write operation needs the forward current and the injection efficiency is low. PA0 (3) The reciprocal action between multiple memories is strong because a forward current is used. PA0 (4) The breakdown voltage of the source and drain regions is low because the doping density is high. PA0 (5) Multiple memories do not have uniform characteristics because the memory characteristics depend strongly on the pattern of the memory.
Therefore, a bipolar-electron-injection type memory is not useful even though there is a good characteristic in that the write voltage is low.